Piezoelectric crystal apparatus



Apri@ la 1949., .3. M. WQLFSKHLL 29467935 PIEZOELECTRC CRYSTAL APPARATUSApri! 32 3949 J. WQLFSMLL v PEZOELECTRI C CRYSTAL APPARATUS Filed Feb.16J 1939 3': Sheets-$116421; 3

la' Lf LA ATTORNEY Patented Apr. l2, 1949 UNITED STATES PATENT OFFICEPIEZOELECTRIC CRYSTAL APPARATUS John M. Wolfskill, Erie, Pa.

Application February 16, 1939, Serial No. 256,781

(Cl. ril-327) 2s claims. 1

This invention relates to piezo-electric crystals and apparatus foremploying piezo-electric crystals, generally. More particularly thisinvention relates to piezo-electric crystals and apparatus for employingpiezo-electric crystals adapted for generating ultra-high frequencyelectric oscillations.

This application is a continuation in part of my application Serial No.38,134 led August 27, 1935, issued as Patent No. 2,157,808.

An object of this invention is to provide a piezo-electric crystaladapted for harmonic operation Another object of this invention is toprovide a piezo-electric crystal adapted to oscillate emciently atfrequencies which are materialy higher than its fundamental frequency.

A further object of this invention is to provide a piezo-electriccrystal adapted to sustain oscillations which bear a harmonic relationto its fundamental frequency.

Still a further object` of this invention is to provide a piezo-electriccrystal of quartz ground to a thickness such that the fundamentalfrequency of the crystal is in the neighborhood of 4 megacycles andoperating the crystal on a harmonic of the fundamental frequency forstabilizing the frequency of electric oscillations of said harmonicfrequency.

Still another object of this invention is to provide a piezoelectriccrystal oscillator adapted to lstabilize the frequencies of oscillationsof ultra-high frequencies.

Still another object of this invention is to provide a piezo-electriccrystal cut and ground in such a manner that the crystal can be made tooscillate at frequencies which bear an harmonic relation to the lowestfrequency at which the crystal will oscillate.

- Still another object of this invention is to provide a crystaloscillation generator adapted to oscillate readily and efficiently atany one of a series of progressively increasing high frequencies.

Another object of this invention is to provide piezo-electric crystalapparatus adapted for stabilization, oscillation and absorption of highand ultra-high frequency electrical oscillations, and employing apiezo-electric crystal with a high, approaching an optical, finish, andwith the major faces thereof being slightly convex.

A further object of this invention is to provide piezo-electric crystalapparatus adapted for stabilization, generation and absorption of highand ultra-high frequency electrical oscillations,

said piezo-electric crystal apparatus consisting of a crystal elementwith the major faces thereof slightly convexv and with a high'nishapproaching an optical finish and a holder for said element, said holderhaving at least one electrode thereof of a small area compared to thearea of the corresponding major face of the crystal element tofacilitate the use of the crystal apparatus on the high and ultra-highfrequency oscillations. 1

Still a further object of this invention is to provide piezoelectriccrystal apparatus adapted for stabilization, generation and absorptionof high and ultra-high frequency electrical oscillations, saidpiezoelectric crystal apparatus consisting of a crystal element with themajor faces thereof slightly convex and with a high finish approachingan optical finish thereon, said major faces being cut substantiallyparallel to the X- axis of the mother crystal and at an angle between+20 and +40 degrees to the optic axis.

A further object of this invention is to provide piezoelectric crystalapparatus adapted for use with high and ultra-high frequency electricaloscillations and employing a piezoelectric crystal adapted to oscillatein the thickness mode.

. said crystal having at least one of its major faces of approximatelyone square inch area and slightly convex, the thickness of said crystaladjacent to the edges thereof being approximately 75 millionths of aninch less than the thickness in the central portions of said crystal.

A further object of this invention is to provide a piezoelectric crystaladapted to oscillate in the thickness mode, said crystal having at leastone of its major faces slightly convex and being characterised by theabsence of slight depressions in the contour of its major faces.

.Another object of this invention is to provide a method forfacilitating harmonic operation of piezoelectric crystal apparatusincluding the steps of placing an electrode which is relatively smallvcompared to a major face of the crystal, over said major face of thepiezoelectric crystal and moving said electrode over the surface of saidmajor face until said electrode is over a portion high and ultra-highfrequencies in a more economical, satisfactory and practical manner thanhas heretofore been possible. My invention has a number of distinctadvantages and features and these will be more fully set forth in thefollowing specification. The principal feature of this invention is thatit facilitates and greatly simpliiles the production and use ofpiezoelectric crystals on high and ultra-high frequencies. This lhasbeen accomplished by cutting and grinding and lapping the piezoelectriccrystals so that the major faces, that is the electrode faces, arepractically parallel and the central portions of said faces are freefrom even minute depressions. The peripheral portions of the crystalelement are ground down slightly to make at least one of the major facesslightly convex. I have found that grinding down the thickness of thecrystal around the edges should be very slight and that this peripheralthickness should be not more than 0.000075 of an inch less than thethickness in the center in the case of a crystal that is approximately0.7 inch wide, 0.9 inch long and has a third harmonic of around 15megacycles. Crystals found to be the best harmonic oscillators were cutfrom the mother crystal with the major faces of the crystalsubstantially parallel to the X-axis and at various angles between plus20 degrees and plus 40 degrees to the optic axis.

My invention greatly simplies piezoelectric quartz crystal controlledoscillation generation inasmuch as it eliminates the necessity ofemploying frequency doublers and frequency multipliers. This is amaterial advantage since losses in frequency doublers and multipliersare enormous when working with oscillations having frequencies higherthan 30 megacycles, for example. My inventi-on readily producesoscillations higher than 30 megacycles which are frequency stabilized,without requiring the use of any frequency doublers and multipliers.This is accomplished by using a crystal element adapted to vibratestrongly on harmonic frequencies. .I prefer to employ a transverse orshear vibration, and in so doing greatly increase the strength of thevibrations corresponding to the odd harmonics. Vibrations other than thetransverse or sheer type may be employed.

an absorption wavemeter on its harmonics for calibration purposes. Thisis accomplished by coupling the crystal plate to the electrical network,oscillator or receiver through an inductance or condenser.so thatabsorption takes place on the harmonic frequencies of the crystal, andthe electrical network, oscillator or receiver may be frequencycalibrated as the frequency of the electric current fed to theelectrical network or receiver or taken from the oscillator is variedthrough the rangeof the harmonics of the crystal plate and the poweroutput of the electrical network, receiver or oscillator is measured orindicated. Sharp indications of electric power ab- By harmonics orharmonic frequencies I mean various frequencies called overtones whichare approximately integral multiples of the fundamental frequency. Thus,odd harmonics would be approximately odd multiples of the fundamentalfrequency. By selecting odd harmonics such as the third, fth, seventh,ninth, eleventh, and so on of, for example, a crystal that willoscillate on a fundamental frequency of 7.5 megacycles, vibrations ofultra-high frequencies such as 22.5 megacycles corresponding to thethird harmonic, 37.5 megacycles corresponding to the fth.' 52.5megacycles corresponding to thegseventh harmonic, 67.5 megacyclescorresponding to the ninth harmonic, and so on, may be obtained directlyfrom the crystal oscillator.

In this manner the crystal vibrating on the harmonic can be used as theprimary source of oscillations when employed in a conventional vacuumtube circuit, will not necessarily be simply a stabilizing device in aself-oscillatory circuit.

Another feature of this invention is that the crystals or piezoelectricbody also may be operated on its harmonics when used as a resonatorproducing some reaction on an associated network, or it may be used infilter circuits, or as sorption are obtained at the harmonic frequenciesof the crystal plate serving as frequency calibration points for theelectrical network. receiver or oscillator.

Other features and advantages of this invention will be apparent fromthe following specification and the accompanying drawings in which,briey, Figures 1a and 1b illustrate the manner in which the crystal ofmy invention is cut from the quartz hexahedron; Fig. 2 illustrates theorientation of the crystal with respect to the X, Y and Z axes; Fig. 3is a circuit diagram illustrating the crystal connected to an oscillatortube; Figs. 4a and 4b illustrate a crystal element vibrating in theshear mode; Fig. 5 is an enlarged view of the crystal element showingthe reduced peripheral dimensions greatly exaggerated; Fig. 6 is asectional view through a piezoelectric crystal holder adapted for usewith high and ultra-high frequency crystals; Fig. 7 is a sectional viewtaken along the line 'l-'l of Fig. 6; Fig. 8 is a sectional view takenalong the line 8-8 of Fig. 6; Fig. 9 is a sectional view taken along theline 9-9 of Fig. 6, and Fig. 10 is another circuit diagram showing thecrystal connected to a vacuum tube.

Fig. 11 shows in vertical section a modied form of the holder of Fig. 6having two small button electrodes;

Fig. 12 shows in vertical section a further modifled form of the holderof Fig. 6 provided with a plurality of small button electrodes any oneof which may be selectively brought into electrical relation with thecrystal; and

Fig. 13 shows in top plan view with parts broken away the modifiedholder of Fig. l2.

Referring to Figs.y 1a and 1b of the drawing in detail, referencenumeral i designates a section of a quartz hexahedron from which a plateor slab 2 adapted to be used as a piezo-electric crystal, is cut. Theposition of the plate 2 in the quartz hexahedronal crystal is shown inFig. la, with respect to the :c or electric axis and the y or mechanicalaxis. Fig. 1b shows the angle at which the crystal plate is cut withrespect to the a or optic axis. In Fig. la the quartz hexahedron isshown with the optic or z axis at right angles to the .1: and y axeswhich are illustrated in the same plane. Fig. 1b, however, shows the yand z axes in the same plane and the :c axis at right angles to thatplane. The plate 2 is cut from the crystal l in the manner shown, withthe principal surfaces 3 and 4 of the plate, cut at an angle 0 withrespect to the optic or a axis. The principal surfaces 3 and 4 are cutsubstantially parallel to the electric or n: axis.

The orientation ofthe crystal plate 2 with respect to thecrystallographic axes y and .e is illustrated in Fig. 2, wherein thesubstantially parallel relation of the sides 3 and 4 to the n:

axis, and the angle between these sides and the z axis, is llshown.

The values of the angle 0 may vary over rather wide limits and I havefound that a piezoelectric plate suitable for use as a high andultra-high frequency harmonic vibrator may be obtained when the plate isout at an angle 6 anywhere between plus degress and plus 40 degrees;however plus degrees is the optimum angle, although an angle ofsubstantially plus degrees may also be used producing a crystal platehaving a low frequency-temperature co-efllcient, the harmonic frequencychanging not more than three cycles in a million per degree centigradetemperature change.

lThere are two kinds of quartz, namely, righthanded and left-handed. Inthe case of righthanded quartz, the convention is adopted that apositive angle is a clockwise rotation of the principal axis (opticaxis) when the electrically positive face as determined by a squeeze, isup. For left-handed quartz, a positive angle is a counterclockwiserotation of the principal axis when the electrically positive face isup.

After the slab 2 is cut from the quartz crystal l it is ground so thatits electrode faces 3 and i are given a high finish that approaches anoptical finish. The aforesaid major or electrode faces are slightlyconvexed so that the peripheral portions of a crystal dll inch Wide and0.9 inch long, adapted to oscillate on a third harmonic of around 15megacycles, are about 'I5 millionths of an inch thinner than the centralportions of the crystal plate. This convexing is shown greatlyexaggerated in Fig. 5. lin this gure it appears that the corners of thecrystal plate are greatly reduced in thickness over the intermediateparts; this however is further exaggerated although it is of courseobvious that the corners will be the thinnest part of a remitangularcrystal plate.

In grinding these high and ultra-high frequency harmonic crystals I havefound that a feature of utmost importance is to avoid forming evenminute depressions in the surfaces of the major faces. any depressionsformed in the crystal face, then it is extremely diiicult to produceharmonic oscillations in the crystal. For this reason it is desirable,among other considerations, to make at least one of the electrodes, 6,6a of a size considerably smaller than the corresponding crystal face.Where possible it is preferable to make both of these electrodes ofreduced size, that is 1A to 1/2 inch in diameter for the crystaldimensions previously given, Other considerations in employingelectrodes of reduced size with harmonic piezoelectric oscillator platesare that the damping effect of the electrode is reduced and the crystaloscillates more freely. The electrostatic capacity is also reduced andthe radio frequency voltage developed per unit crystal area is increasedso that the current density in the crystal is increased. As a resultharmonic oscillations start more readily in the crystal.

For connecting the piezoelectric plate to an electric circuit to produceelectrical oscillations, a crystal holder, such as illustrated in Figs.6, 7, 8 and 9, is employed. This holder is of the plugin type having acasing l1 of insulation material designed for high and ultra-highfrequencies. A pair of contact pins I9 and 2l) are attached to thecasing and a cover 6 is held on the casing by small bolts i8. This coveralso functious as one of the crystal plate electrodes.

The cover 6 contacts the crystal plate 2 over the entire correspondingface of the crystal plate, however, where desired the bottom face of thecover E next to the crystal may be turned down as in a lathe so that asmall button electrode is formed substantially in the center of thebottom surface of the cover corresponding to the button electrode 6awhereby only small areas of each of the crystal faces are in contactwith the respective electrodes. Such an arrangement is shown in Fig. 1lin which the face of cover 6i is turned down to form the annular recess62 leaving thc If the electrode 6a is placed over central button 63,which constitutes the contacting area of the upper electrode for crystal2. Centering retaining means such as ribs 25 shown in Figs. 6-9 may beprovided.

In the arrangement of Figures 6, 7, 8, 9, and also in that of Fig. 1l.,the cover 8 is connected to the pin 2li through the bolt I8, thethreaded member 22 embedded in the casing and the connector 2i whichengages the member 22 and the pin 20. A piece of braided, flexiblecopper wire 24 is attached to the button @a by soldering 0r otherwiseand this is employed to connect this electrode to the pin i9. Both pinsi9 and 20 are firmly embedded in the bottom of the casting il at one endto hold them in place and this may be accomplished during moulding ofthe casing. A coil spring 23 is provided to hold the button 6a againstthe crystal plate 2 and to provide a slight pressure against theaforesaid crystal plate, This coil spring is positioned substantially inthe center of the cavity 2a, formed in the casing il, between theadjacent ends of the ribs 25 extending into said cavity from the fourcorners thereof. These ribs are cut away slightly to form shoulders forreceiving the button electrode 5a rather loosely therebetween andfacilitate assembly of the piezoelectric crystal unit.

A form of circuit that may be employed to produce harmonic frequencyoscillations of high or ultra-high frequencies from the piezoelectriccrystal plate is illustrated in Fig. 3 wherein the three element vacuumtube 5 is connected with its grid electrode 8 to the crystal holderelectrode 6a and the cathode 5 to the holder electrode 6. A grid leakresistor 'i is connected between the grid and. the cathode. The anode I0is connected to the oscillatory circuit il consisting of the inductancei2 and the variable condenser i3. A source of anode current supply i4shunted by a high frequency by-pass condenser l5 is connected to theoscillatory circuit il and to the cathode s.

The vacuum tube 5 may be of the multiple grid type and the connectionsthereto may be as schematically illustrated in Fig. 10. However, in thecase of the three element tube it is very desirable to employ a vacuumtube having low internal capacities between the electrodes thereof andfor this as well as other reasons type 955, 6E6, 6J5G and RK34 tubes arepreferred. In the case of three element vacuum tubes of the typeillustrated in Fig. 3 the capacity Cg between the cathode and the gridshould not exceed three or four micro-microfarads at the harmonicfrequency on which the crystal is oscillating. The grid to anodecapacity Cp and the anode to cathode capacity Cep also should becorrespondingly small. By keeping the interelectrode capacities in thetube down to low values, to reduce the by-passing action of thesecapacities for the high and ultra-high frequency electrical oscillationsproduced in the piezoelectric crystal plate, the load on thepiezoelectric crystal plate is reduced for any particular gridelectrodeexcitation derived from the crystal. This is an importantconsideration when high and ultra-high frequency electrical oscillationsare produced in the crystal because the capacitative susceptance of evensmall capacities is by no means negligible at these frequencies and if asubstantial capacity exists between the cathode and the grid arelatively large radio frequency current may flow through this capacitycausing an additional load to be placed upon the [crystal and therebyreducing the radio frequency potential of the crystal to a point that itis insufficient properly to drive the vacuum tube at the desiredharmonic frequency of the crystal.

The oscillatory circuit II is adjusted substantially to the harmonicfrequency of the crystal desired either by varying the inductance I2 orthe capacity I3 until this circuit is resonant substantially to theharmonic frequency desired.

In the case of the circuit shown in Fig. 3 the ratio of the capacity tothe inductance in the circuit il should be high for proper operation ofthe crystal on a harmonic.

The circuit shown in Fig. also may be used very conveniently withharmonic crystal oscillator plates of my invention. This circuit employsa radio frequency pentode tude 23. which may be an 802 or RK23 type, andthe crystal 2 positioned between the electrodes 6a and 8. is connectedto the grid electrode 8a and the indirectly heated cathode 9a. Thecathode oscillatory circuit I la consisting of the inductance l2a andcondenser 43a is tuned substantially to one of the harmonics of thecrystal plate 2 and this crystal plate is made to vibrate or oscillateat that harmonic frequency. A positive potential is applied from thesource 33 through the connection 34 to the grid electrode 21 whichfunctions more or less as an anode for the crystal oscillator tubecircuit. The source 33 which may be a battery, generator, rectifiedalternating cursisting of approximately 30,000 ohms and a. radiofrequency choke 1b of approximately 2.5 millihenries are connectedbetween the grid 8a and the low potential end of the oscillatory circuitIIa. The oscillatory circuit 30 consisting of an inductance 3| andvariable condenser 32 is connected between the positive terminal of thecurrent'source 33 and the anode 29 of the tube 26. This oscillatorycircuit is also of a high capacity to inductance ratio and in this casethe resonant frequency is adjusted to be substantially a multiple of theresonant frequency to which the circuit IIa is adjusted. The circuit Ilais adjusted substantially to an odd harmonic of the crystal 2 so thatthe crystal oscillates at this harmonic fre-,

quency and the circuit 30 is adjusted to a multiple of this harmonicfrequency so that an output circuit coupled to the circuit 30 hasimpressed thereon electrical oscillations of this multipled frequency.This circuit is particularly valuable .in ultra-high frequency circuitswhere it is desired to obtain piezoelectric crystal frequency controland stabilization.

In order to facilitate initiating and maintaining the crystal plate 2 inoscillation on a harmonic frequency, such as, the third, fifth, seventhand so on, the electrode 6a, shown in Fig. 5, for example, may be movedover the face of the crys- B the desired harmonic found that the plateoscillates best at a harmonic frequency when the electrode 6a is ofrelatively small diameter, between 1A inch and 1/2 inch, and is locatedover a portion of the crystal face that is free from minute depressions.Means, such as, a. manually operable screw device or a plurality ofsmall button electrodes connected together and positioned side by side,any one or more of which may be raised from or lbweredA to the electrodeface of the crystal plate by suitable screws, may be provided to theholder for use with harmonic crystal if desired. Such an arrangement isshown in Figs. 12 and 13 in which metal cover plate 83 closes the tcp ofcylindrical insulating material casing ll whose bottom is closed bymetal plate 84 which constitutes the bottom electrode upon which crystal2 rests. A plurality of screws, 83, 90, 0l. @2, d3, pass through coverplate 83 and at their lower ends carry small button electrodes as 9d,95, 96. Mounting pin 30 makes connection with bottom plate dil. `Mounting pin 'I3 is mountedon bottom plate Btl insulated therefrom byinsulating plug 0G, and is connected by wire 3l and bolt d3 to coverplate Any desired one of the button electrodes carried by screws 33, 90,0i, 02, d3, may be selectively lowered into electrical relation withcrystal 2, or may be withdrawn upwardly therefrom. Screw i8 is insulatedfrom both cover plate 83 and bottom plate 35. Best results are obtainedwhen both electrodes t and 6a are made of re- -duced size, asillustrated in the ease of electrode 6a, with respect to the size of thecorresponding crystal plate faces. However, satisfactory thickness modeharmonic operation is obtained when only one of the electrodes ofreduced size is employed.

It will be observed that I have described various features of anembodiment of my invention in detail, however, I do not desire to limitmy invention to the exact details set forth except insofar as thosedetails may be defined by the claims.

What I claim is as follows:

1. Piezoelectric crystal apparatus adapted for operation on highultra-high radio frequencies, comprising a piezoelectric crystal plateadapted to oscillate on odd harmonics of its fundamental frequency, saidcrystal plate having a pair of substantially parallel electrode faces,one of said electrode faces being slightly convex and both of saidelectrode faces having areas free from even minute depressionsdetrimental to harmonic oscillation, a holder for said crystal platecomprising a pair of substantially flat electrodes for said electrodefaces, one of said electrodes for said slightly convex electrode facehaving an area of only a fraction of the area of said slightly convexelectrode face, said electrode being positioned over an area of saidface free from said depressions to facilitate harmonic' operation.

2. Piezoelectric crystal apparatus as set forth in claim 1y furthercharacterized in that said electrode faces of said piezoelectric crystalplate are disposed at an angle of substantially plus 30 degrees withrespect to the optic axis and being substantially parallel to theX-axis.

V3. Piezoelectric crystal apparatus as set forth in claim 1 furthercharacterized in that said electrode faces of said piezoelectric crystalplate are cut at an angle between plus 20 degrees and plus 40 degrees tothe optic axis and being substantal plate until the plate oscillatesvigorously at tially parallel to the X-axis.

frequency. It has been v 4. Piezoelectric crystal apparatus as set forthin claim 1 further characterized in that said slightly convex electrodeface of said-piezoelectric crystal plate is convex approximately '75millionths of an inch.

5. Piezoelectric crystal apparatus as set forth in claim 1 furthercharacterized in that said electrode faces of said piezoelectric crystalplate are finished substantially to an optical finish and said slightlyconvex face is convex approximately 75 millionths of an inch.

6. A method of operating a crystal at an odd harmonic in a shear modewhich comprises `so cutting the crystal that its electrode faces areparallel to the X-axis and make an angle between +20 and +40 degreeswith the Z-axis, convexing at least one of said faces to a slight extentto facilitate harmonic operation and applying an electric field of afrequency corresponding to the desired shear mode harmonic only to asmall portion of the face area of said convex face.

7. A method of operating a crystal at an odd harmonic in a shear modewhich comprises so cutting the crystal that its electrode faces areparallel to the X-axis and make an angle of substantially +30 degreeswith the Z-axis, convexing at least one of said faces to a slight extentto facilitate harmonic operation and applying an electric field of afrequency corresponding to the desired shear mode harmonic only to asmall portion of the face area of said convex face.

8. Piezoelectric crystal apparatus as set forth in claim 1 furthercharacterized in that said electrode faces of said piezoelectric crystalplate are disposed at an angle of substantially +35 degrees with respectto the optic axis and substantially parallel to an electric axis, saidcrystal having on its harmonic frequency a frequency temperaturecoeflicient of substantially less than three parts per million perdegree centigrade.

9. Piezoelectric crystal apparatus, comprising:

a housing of insulation material having an open cavity therein, arelatively small disc-like electrode positioned in said cavity, a springsubstantially centrally positioned in said cavity between said electrodeand the bottom of said cavity for supporting said electrode, a pluralityof upright members projecting from the bottom of said cavity to confinesaid spring therebetween, shoulders formed on said upright members nearthe free ends thereof, the space between said shoulders being just bigenough to receive said electrode ly- .ing fiatwise thereon, apiezoelectric crystal element positioned in the open end of said cavity,said spring urging said electrode against the lower surface/of saidpiezoelectric crystal element and.

tending to force saidcrystal out of said cavity through the opening, asecond electrode and means for securing said second electrode over theopening of said cavity against the outer surface of said crystalelement.

10. Piezoelectric crystal apparatus, comprising: a housing of insulationmaterial having an open cavity therein, a piezoelectric crystal plate, aplurality of upright members extending into said cavity from the bottomthereof to define a central ment of said electrode. a second electrodeand means for securing said second electrode over the opening of saidcavity against the outer surface of said crystal plate.

11. Piezoelectric crystal apparatus, comprising: a housing of insulationmaterial having an open cavity therein, a piezoelectric crystal and arelatively small disc-like electrode positioned in said cavity, saidelectrode having an area equal to a small fraction of the area of themajor face of said piezoelectric crystal adjacent thereto, a pluralityof web-like members projecting from the bottom of said cavity forpositioning said small disc-like electrode substantially in the centerof a major face of said piezoelectric crystal and substantiallyconfining said electrode in said position, a second electrode engagingthe other maior face of said piezoelectric crystal and for closing themouth of said` cavity and means for urging said small disc-likeelectrode against said piezoelectric crystal and said piezoelectriccrystal'against said second electrode.

12. Piezoelectric crystal apparatus, comprising: a housing of insulationmaterial having an open cavity therein, a piezoelectric crystal and arelatively small disc-like electrode positioned in said cavity, saidelectrode having an area equal to a small fraction of the area of themajor face of said piezoelectric crystal adjacent thereto, a pluralityof web-like members projecting from the bottom of said cavity forpositioning said small disc-like electrode substantially in the centerof a major face of said piezoelectric crystal and substantiallyconfining said electrode in said position, a second electrode engagingthe other maior face of said piezoelectric crystal and for closing themouth of said cavity, said second electrode `having a small buttonportion of an area equal to a small fraction of the area of a major faceof said crystal and being positioned on a side of said crystal oppositeto said first mentioned electrode,

and means for urging said small disc-like electrode against saidpiezoelectric crystal and said piezoelectric crystal against said secondelectrode. 13. Piezoelectric crystal apparatus, comprising: a housing ofinsulation material having an open cavity therein, a piezoelectriccrystal and a relatively small disc-like electrode positioned in saidcavity, said electrode having an area equal to a small fraction of thearea of the major face of said piezoelectric crystal adjacent thereto,means for positioning said small disc-like electrode substantially inthe center of a major face of said piezoelectric crystal andsubstantially confining said electrode in said position, a secondelectrode engaging the other major face of said piezoelectric crystaland for closing the mouth of said cavity and means for urging said smalldisc-like electrode against said piezoelectric crystal and saidpiezoelectric crystal against said second electrode.

14. Piezoelectric crystal apparatus, comprising: a housing of insulationmaterial having an open cavity therein, a piezoelectric crystal and arelatively small disc-like electrode positioned in said cavity, saidelectrode having an area equal to a small fraction of the area of themajor face of said piezoelectric crystal adjacent thereto, means forpositioning said small disc-like electrode substantially in the centerof a major face of said piezoelectric crystal and substantiallyconfining said electrode in said position, and a second electrodeengaging. the other major face of said piezoelectric crystal, saidsecond electrode agences having a small button portion of an area equalto a small fraction of the area of a major face of said piezoelectriccrystal and being positioned on a side of said piezoelectric crystalopposite to said first mentioned electrode.

15. Piezoelectrlc crystal apparatus, comprising: a housing of insulationmaterial having an open cavity therein, a piezoelectric crystal and arelan tively small disc-like electrode positioned in said cavity, saidelectrode having an area equal to a small fraction of the area of themajor face of said piezoelectric crystal adjacent thereto, means forpositioning said small disc-like electrode substantially in the centerof a major face of said piezoelectric crystal and substantiallyconfining said electrode in said position, a second electrode engagingthe other major face of said piezos electric crystal and for closing themouth of said cavity, said second electrode having a small huta tonportion of an area equal to a small fraction of a major face of saidcrystal and being posie tioned on a side of said crystal opposite tosaid first mentioned electrode, and means for urging said small discdikeelectrode against said piezo electric crystal and said piezoelectriccrystal against said second electrode.

16. A piezoelectric crystal arrangement which will function in thethiclrness mode of the crystal to give a single frequency response whensubjected to high frequency comprising a piezoelece tric crystal havinga high ratio of dimensional difference between any lateral dimension andits thickness, two electrodes arranged each side of said crystal, thearea of said electrodes being smaller than onemhalf the area of saidcrystal whereby multiple wave phenomena are eliminated.

17. A piezoelectric crystal arrangement which will function in thethickness mode of the crystal to give a single frequency response whensuh-J jected to high frequency comprising a piezoelec tric crystalhaving a high ratio of dimensional difference between its least lateraldimension and its thickness, two electrodes arranged each side of saidcrystal, the area of said electrodes being smaller than onewhalf thearea of said crystal whereby multiple wave phenomena are *eliminated.

i8. A piezoelectric crystal arrangement which will function in thethickness mode of the crystal to give a single frequency response whensubjected to high frequency, comprising a piezo electric crystal havinga high ratio of dimensional difference between the shortest element of amaior face of said crystal passing through the center of said majorface, and the thickness of said crystal, and two electrodes, onearranged on each side of said crystal, in electrical relation with majorfaces thereof respectively, the area of each of said electrodes beingsmaller than one-half .the area of each of said maior faces respectivelyof said crystal, whereby multiple wave phenomena are eliminated andharmonic operation is facilitated.

19. A piezo-electric crystal arrangement which will function in thethickness mode of the crystal to give a single frequency response whensubjected to high frequency, comprising a substantially rectangularpiezo-electric crystal having a high ratio of dimensional differencebetween the shortest element of a major face of said crystal passingthrough the center of said major face, and the thickness of saidcrystal, and two substantially circular electrodes, one arranged on eachside of said crystal, in electrical relation with major faces thereofrespectively, the area la of each of said electrodes being smaller thanone-half the area of each of said major faces respectively, wherebymultiple wave phenomena are eliminated and harmonic operation is facilitated.

20. A piezo-electric crystal arrangement which will function in thethickness mode of the crystal to give a single frequency response whensubjected to high frequency, comprising a substantially rectangularpiezo-electric crystal having a high ratio of dimensional differencebetween the shortest element of a major face of said crystal passingthrough the center of said major face, and the thickness of saidcrystal, and two substantially circular electrodes, one arranged on eachside of said crystal, in electrical relation with major faces thereofrespectively, the diameter of said electrodes being not greater thanone-half the minimum transverse element of the correspending major facepassing through the center of said major face, whereby multiple waveplienomena are eliminated and harmonic operation is facilitated.

2l. ln combination, a piezo-electric crystal adapted to operate in thethickness mode at a frequency which is substantially a harmonic of itsfundamental frequency, said crystal having a high ratio between itsshortest facial dimension through the center of the major faces of saidcrystal andthe crystal thickness, a pair of electrodes positioned inopposing relation, one on each side of said crystal in contact with thecentral area of the major face of said crystal, said area being activeto excite said crystal and being a portion less than one-half of thetotal area of said major face, and connections to said electrodes forapplying oscillations of said haru rnenic frcduency between saidelectrodes to excite said crystal at said harmonic frequency.

22. In combination, a piezoelectric crystal adapted to operate in thethickness mode at a frequency which'is substantially a harmonic of itsfundamental frequency, said crystal having a high ratio between itsshortest facial dimension through the center of the major faces of saidcrystal and the crystal thickness, a pair of electrodes positicned inopposing relation, one on each side of said crystal in contact with thecentral area of the major face of said crystal,

y said central area being active to excite said crystal, each electrodebeing substantially sur rcunded by uncovered area of the respectivemajor face, said uncovered area of said major face being free in spaceand greater than the area ci said electrodes thereby materially reducingcapacity between said electrodes at the fre quency of said harmonic, andconnections to said electrodes for applying oscillations of said han1monic frequency between said electrodes for ex citation of said crystal.

23. in combination, a piezoelectric crystal adapted to operate in thethickness mode at a frequency which is substantially a harmonic of itsfundamental frequency, said crystalhaving a high ratio between itsshortest facial dimension through the center of the major faces of saidcrystal and 'the crystal thickness, a pair of electrodes positioned inopposing relation, one on each side of said crystal cooperating with thecentral area ofthe major face of said crystal, said central area beingactive to excite said crystal, at least one of said electrodes beingsmall relative to the area of the corresponding maior face of saidcrystal and in contact with said major face and being substantially sur@rounded by uncovered area of the respective major face of said crystalgreater than the area of said small electrode, whereby capacity betweensaid electrodes is reduced, and connections between said electrodes forapplying oscillations of said harmonic frequency between said electrodesfor excitation of said crystal.

24. In a piezoelectric crystal holder, a flat piezoelectric crystal, ahousing comprising an electrically conducting plate of area at least asgreat as a major face of said crystal, said plate being on a rst sideofsaid housing, said crystal being positioned with a rst major facethereof engaging said platea button electrode of area not exceedingone-half the area of a major face of said crystal positioned on the sideof said crystal opposite said plate, a spring positioned between saidbutton electrode and a second side of said housing opposite said iirstside thereof and urging said button electrode against the second majorface of said crystal, and means comprised in said housing for retainingsaid button electrode and said spring in substantially fixed relation tosaid housing.

25. A holder as recited in claim 24, said crystal being mountedsubstantially centrally in said housing, and said retaining meansholding said button electrode and said spring substantially centrally insaid housing.

26. In a piezoelectric crystal holder, a fiat lpiezoelectric crystal, asupport comprising an electrically conducting plate of area at least asgreat as a major face of said crystal, said plate having a raisedelectrically conducting button portion thereof formed on one sidethereof and of area substantially less than the full area of said plate,said crystal being positioned with a first major face thereof engagingsaid raised button portion, a button electrode positioned on the secondmajor face of said crystal opposite said plate, said raised buttonportion and said button electrode being respectively substantially lessin area than the area of a major face of said crystal, and holding meansfor holding said button electrode adjacent said major face of saidcrystal.

27. In a piezoelectric crystal holder, a flat piezoelectric crystal, asupport comprising an electrically conducting plate, said crystal beingpositioned with a first major face thereof engaging said plate, aplurality of small button electrodes each of area relatively small withrespect to the area of a major face of said crystal, and mouuting meanscarried by said support and comprising selective control means forselectively positioning any desired one of said button electrodesagainst the second major face of said crystal opposite the first majorface thereof so engaging said plate.

28. In a piezoelectric crystal holder, a at piezoelectric crystal, ahousing comprising an electrically conducting plate, said plate being ona first side of said housing, said crystal being positioned with a rsi;major face thereof engaging said plate, a button electrode of area notexceeding one-half the area of a maior face of said crystal positionedon the side of said crystal opposite said plate, a spring positionedbetween said button electrode and a second side of said housing oppositesaid first, side thereof and urging said button electrode against thesecond maior face of said crystal, and means comprised in said housingfor retaining said button electrode and said spring in substantiallyfixed relation to said housing.

JOHN M. WOLFSKILL.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,017,897 Emerslaben Oct. 22,1935 2,119,848 Hawk June 7, 1938 2,128,837 Meahl Aug. 30, 1938 2,151,754Fair Mar. 28, 1939 2,159,796 Hawk May 23, 1939 2,173,589 Mason Sept. 19,1939 2,178,224 Diehl Oct. 31, 1939 2,218,200 Lack et al. Oct. 15, 19402,249,933 Bachmann July 22, 1941 2,343,059 Hight Feb. 29, 1944 FOREIGNPATENTS Number Country Date 279 595 Great Britain Nov. 3, 1927 457 ,342Great Britain Nov. 26, 1936 OTHER REFERENCES Proceedings of theInstitute of Radio Engineers, vol. 14, Nov. 4, August 1926, page 451.

